A plasma display panel (PDP) has received widespread attention as a color image display device having a large screen. PDPs employ phosphors of three colors of green, red, and blue as light-emitting material, which are emitted by ultraviolet excitation. However, only a few of the materials satisfy required conditions, such as luminous intensity, and color purity. In a real-time situation, manufacturers have been searching for a desirable combination of green, red, and blue phosphor materials that entirely satisfies the conditions.
Among of all the conditions, afterglow characteristics of phosphors considerably depend on the color, particularly, there is a big difference between the phosphor emitting blue and the phosphor emitting green. For example, BaMgAl10O17:Eu, which is a typical blue phosphor, exhibits afterglow that lasts a few μs, whereas Zn2SiO4:Mn used for a green phosphor has long-lasting afterglow close to a TV field of approx. 16.7 ms (where, the afterglow-lasting time is represented on a scale of one tenth). Due to the difference in afterglow-lasting time, a moving image is sometimes accompanied by an unintended color that was not included in the original image. More specifically, when a bright point moves in an image, a green “tail” appears behind the bright point, or in another case, when the screen changes into dark, a green image remains on the screen. In particular, skin of a figure image accompanied by a green tail with high relative luminous efficiency is an eyesore, deteriorating image quality.
To address the deterioration in image quality due to the difference in afterglow-lasting time, there are some suggestions. For example, with respect to an image signal corresponding to the color emitted by the phosphor having a short afterglow time, an additional image signal (hereinafter referred to as a pseudo afterglow signal) is generated, and the pseudo afterglow signal is added to the original image signal so as to correspond with the length of afterglow of other colors. One of the suggestions introduces a method of improving the image quality by adding one-field-before image signal as the pseudo afterglow signal to the current-field image signal at a uniform rate (for example, see Japanese Patent Unexamined Publication No. 2002-14647).
However, the method above—the image signal of the previous field is simply added as the pseudo afterglow signal to the image signal of the current field at a uniform rate—can invite inconveniences. For example, in the display having a bright, white window pattern moving against a dark background, color difference generated in the area having afterglow can increase mismatch feeling in color change. The mismatch rests on the fact that natural afterglow can be seen with exponential attenuation as it goes away from the window pattern, whereas the pseudo afterglow, which is generated by adding one-field before image signal to the image signal of the current field, is no longer dependent on the distance from the window pattern.
It is the object of the present invention to provide a method and apparatus of image signal processing capable of maintaining the color of afterglow the same as that of the original image to improve image quality even in the image display using a color image display device having a plurality of light-emitting materials having a difference in afterglow-lasting time.